BODY FLUIDS AND CIRCULATION 113 



and Busycon and in the hypobranchial gland of Murex. It is believed that 

 serotonin is the normal mediator of the cardiac excitatory nerves of 

 Mercenaria. Cephalopods contain much amine oxidase, an enzyme capable 

 of oxidizing serotonin, tyramine and related compounds (8, 11, 12, 13, 37, 

 44, 46, 97, 100, 103, 104, 105, 108, 129, 134, 135, 136, 141). 



Holothuria. The intestinal vessels of holothurians are weakly contrac- 

 tile, and show localized peristaltic waves. By analogy with the pharmacol- 

 ogy of other hearts, contractions of the intestinal vessel of Stichopus 

 appear to be myogenic. It beats spontaneously, and is inhibited by acetyl- 

 choline in rather low concentrations (threshold 10~ 14 ). Eserine potentiates 

 the response to acetylcholine, and nicotine inhibits vascular contractility. 

 The action potential of the intestinal vessel appears to be a simple wave of 

 negativity (109). 



Neurogenic Hearts 



Arthropods. Neurogenic hearts are best exemplified by arthropods. In 

 this type the rhythm of the heart beat is controlled by the nervous system. 

 Among some of the lower Crustacea (Cladocera) the heart appears to be 

 myogenic, whereas neurogenic hearts are characteristic of higher forms. 

 During development the larval heart shows myogenic automaticity, pulsat- 

 ing rhythmically before the advent of the nervous system (lobster, king 

 crab). Later, the nervous system predominates in control of movement, 

 although it is reported that even the deganglionated heart of adult Limulus 

 displays myogenic automatism, but the rhythm is slower and the beat is 

 peristaltic in character (75, 108). 



In the hearts of Malacostraca and Xiphosura {Limulus) there are nerve 

 cells in which the excitatory process originates. These intrinsic cardiac 

 neurones are termed the local cardiac system by Alexandrowicz. They 

 consist of a small and fixed number of nerve cells situated along the course 

 of a ganglionic trunk in the dorsal wall of the heart (Malacostraca) (Fig. 

 3.17). Small and large neurones are distinguished in most groups: there 

 are six neurones in isopods (Ligia) ; sixteen in stomatopods ; nine in deca- 

 pods; five or six in the cardiac trunk of amphipods and mysids. The 

 axonal processes of these cells proceed to the heart muscles. This local 

 system is connected to the central nervous system by extrinsic cardiac 

 nerves. In addition, the arterial valves and alary muscles receive a separate 

 innervation of their own from the c.n.s. In the heart of Limulus there are 

 lateral nervous trunks linked by thin connectives to a median ganglionic 

 trunk (Fig. 3.9). The latter contains scattered small multipolar neurones 

 and large unipolar neurones. 



Various experimental methods have been utilized in studying the 

 activity of the intrinsic nervous system. In Ligia the activity of the heart is 

 ruled by the dorsal ganglionic trunk, and by severing this trunk different 

 pulsatory rhythms are produced in various parts of the heart. It has also 

 been observed that when the cardiac ganglion of Maia is transected, the 

 heart stops beating. In the heart of Limulus excitation originates in the 



